Study Of The Degradation Of Chiral Neonicotinoid Insecticide Cycloxaprid In Aerobic Soils

Cycloxaprid is a kind of neonicotinoid insecticide developed by China with independent intellectual property. It is quite prospective in industrialization for its high activity against imidacloprid-resistant pests. However, in order to ensure the environment and agro-products safety, the degradation of cycloxaprid in soils is needed to be further studied.To evaluate the potential risk of cycloxaprid posed to the environment and human beings, the degradation rate, degradation products and degradation pathway of cycloxaprid in aerobic soils are investigated from the perspective of enantio selectivity using14C-labelled cycloxaprid. Liquid scintillation measurement technology and high-performance liquid chromatography-tandem mass spectrometry are used to trace the degradation products.According to the results of the120h incubation experiment, there was no enantioselectivity in the formation of bound residue and the degradation of cycloxaprid isomers. The rapid disappearance of cycloxaprid in three kinds of typical soils indicated that bound residue developed rapidly and the proportion of extractable residue declined rapidly. The soils in descending order of bound residue proportion were yellow loamy soil, coastal saline soil, red clay soil. Once introduced into the soils, cycloxaprid degraded or disappeared rapidly and was considered to be easily degradable. The disappearance dynamics fitted well to the first-order kinetic equation. The half-lives of cycloxaprid in yellow loamy soil, coastal saline soil, red clay soil were53.32h,77.02h,23.10h respectively. Analysis on correlation between the half-lives and soil physicochemical properties showed that soil pH was one of the most influential factors. The degradation rate in soils followed the order of red clay soil, yellow loamy soil and coastal saline soil.According to the results of the80d incubation experiment, there was no enantioselectivity in the degradation products and pathways of cycloxaprid isomers. Three degradation products were identified after the initial cleavage of oxabridged seven-membered ring, including1-(6-chloronicotinyl)-2-nitromethylene imidazolidine (M1),1-(6-chloropyridinyl)-2-carboxylic acid-imidazoline (M2) and1-(6-chloronicotinyl)-2-hydroxyl-imidazolidine (M3). At the beginning of incubation, the majority of degradation products was Ml. And then Ml gradually transformed into M2and M3with time. The dynamics of the three kinds of degradation products were markedly different from soil to soil. In yellow loamy soil, M1rapidly degraded into M2and M3. In coastal saline soil, Ml preferably transformed into M2and the content of M3was limited. In red clay soil, the degradation rate of the M1into M2and M3was slower than in yellow loamy soil. Based on the structural properties of three kinds of degradation products and the dynamics of them, a possible degradation pathway was proposed. Ring cleavage, denitration, carboxylation, hydroxylation contributed to the cycloxaprid degradation reactions in soils. It was quite different from the pathway in flooded soils. It suggested that the environment that cycloxaprid was applied in would greatly influence the degradation process.